Beyond the edge of the solar system, something has
gradually dragged two of America's oldest space probes -- Pioneer 10 and
Pioneer 11 -- a quarter-million miles off course. Astrophysicists have
struggled 15 years in vain to identify the infinitesimal force at play.
The Pioneer anomaly, as it is called, throws a monkey wrench into celestial
mechanics.

Slava Turyshev may have found
the answer in NASA's trash. Reconstructing decades of discarded spacecraft
data, the Russian-born astrophysicist and the private space enthusiasts
helping him say they believe they are on the verge of solving a mystery
of time and gravity that has perplexed a generation of physicists and might
have confounded Newton and Einstein.

The anomaly officially materialized
in 1988, 16 years after NASA launched Pioneer 10 toward the outer planets.
The 568-pound spacecraft had been designed to stay in radio contact with
Earth just 21 months, time enough for it to become the first spacecraft
to pass through the asteroid belt, the first to fly past Jupiter and the
first to visit the outer solar system. The plutonium-powered probe, however,
transmitted data 31 years until 2003.

As it sped through space, a specialist in radio-wave
physics named John Anderson at NASA's Jet Propulsion Laboratory noticed
an odd thing. The spacecraft was drifting off course. The discrepancy was
less than a few hundred-millionths of an inch per second for every second
of spaceflight, accumulating year after year across billions of miles.
Then Pioneer 11, an identical probe escaping the solar system in the opposite
direction, also started to veer off course at the same rate.

Ordinarily, such small deviations might be overlooked,
but not by Dr. Anderson. He monitored the trajectories six years before
calling attention to the matter. "I'm a little like an accountant," Dr.
Anderson said. "We have Newton's theory and Einstein's theory, and when
you apply them to something like this -- and it doesn't add up -- it bothers
me."

Not everything in the solar system adds up, of course.
The moon's actual orbit is off its calculated course by about six millimeters
a year. No one knows why. The standard yardstick for length on an interplanetary
scale, the Astronomical Unit, grows by about seven centimeters a year.
Scientists have yet to agree on an explanation. At least four recent planetary
probes experienced such unaccountable changes in velocity as they passed
Earth, Dr. Anderson and his colleagues reported this past March in Physical
Review Letters.

None prompted the scrutiny given the Pioneer anomaly.
In hundreds of technical papers, Dr. Turyshev and scores of other space
scientists considered and eliminated most mundane explanations, including
fuel leaks, software bugs, mechanical flaws, navigation errors, fading
plutonium power, planetary influences, the solar wind, even the effect
of ocean tides and local plate tectonics on the placement of ground antennas.
Others proposed more far-fetched scenarios: the tug of dark matter, the
accelerating expansion of the universe or a breakdown of gravity's most
fundamental laws.

Indeed, Dr. Turyshev at the Jet Propulsion Laboratory
and his colleagues around the world regard the Pioneer probes as the largest
test of Newton's law of gravity ever conducted. By that axiom, refined
by Einstein, any two objects in the universe exert gravitational attraction
on each other proportional to their mass and affected predictably by the
distance between them.

"We would expect the two spacecraft to follow Newton's
law of gravity," Dr. Turyshev said, "but they in fact fail to confirm Newton's
law. If Newton is wrong, Einstein is wrong too."

For 14 years, Dr. Turyshev
sought a simpler answer. He finally wondered whether heat radiating unevenly
from the probe might be the cause but lacked enough information.

Then, at JPL in 2002, he discovered 400 computer tapes
of Pioneer data gathering dust under a stairwell. In 2005, he intercepted
70 filing cabinets of Pioneer engineering data on their way to the junk
heap at the NASA Ames Research Center, at Moffett Field, Calif. The computer
files held all of the Pioneer mission data, but they were unreadable.

With no formal NASA funding, almost 6,000 members of
The Planetary Society, a space-exploration advocacy group based in Pasadena,
Calif., donated $220,000 to translate the antiquated data into a digital
format that a modern computer can read. "This is not something that should
be brushed away just because it is old data," said society Executive Director
Louis Friedman. Victor Toth, a noted Canadian computer expert, donated
his time.

After six years of work, the
researchers expect to finish restoring the last data files next month.
Based on a partial analysis, Dr. Turyshev reported in April at a meeting
of the American Physical Society in St. Louis that at least 30% of the
force can be attributed to heat radiating from the probe. "The rest is
unknown," he said.

In the year ahead, Dr. Turyshev and his colleagues
plan to use the vintage data to create a computer flight simulation of
the two Pioneer missions with a precision never before possible. That may
finally lay it to rest.

"There is some hope that this would show a new physics,"
Dr. Turyshev said. "With the Pioneers, we are exploring uncharted territory."

FAIR USE NOTICE: This page contains copyrighted
material the use of which has not been specifically authorized by the copyright
owner. Pegasus Research Consortium distributes this material without profit
to those who have expressed a prior interest in receiving the included
information for research and educational purposes. We believe this constitutes
a fair use of any such copyrighted material as provided for in 17 U.S.C
§ 107. If you wish to use copyrighted material from this site for
purposes of your own that go beyond fair use, you must obtain permission
from the copyright owner.